Multiplex radio signaling system



Jan. 4, 1938. E. H. ARMSTRONG 2,104,012

7 MULTIPLEX RADIO SIGNALING SYSTEM Filed Sept. 14, 1935 5 Sheets-Sheet 1x m m Q 3 & R S N g S g k S 1 Q k t q k 2 a a w m a -a a Q a .g Q & 8Q Qw a "W:

: 2 Q 7 m o I INVENTOR. Edwin HAr/nsrmnq.

ATTORNEYS.

Jan .4, 193s. E. H. ARMS RONG 2,104,012v

MULTIPLE)! RADIO SIGNALING SYSTEM Filed Sept. 14, 1955 5 Sheets-Sheet 2fi ER INVENTOR.

EdW/I? H. Arm srrang BY MM M ATTORNEYS.

Jan. 4, 1938. E. H. ARMSTRONG MULTIPLEX RADIO SIGNALING SYSTEM 5Sheets-Sheet 3 Filed Sept. 14, 1935 ATTORNEYS.

Patented Jan. '4. 19

UNITED STATES PATENT OFFICE 8 Claims.

This invention relates to a new system of multiplex operation in radiosignaling by which it is possible not only to multiplex successfully butto operate over greater distances and more efiectively with a givenamount of power than if the power were divided among separatetransmitters operating on separate simplex channels.

Many methods of multiplexing with frequency modulation have beenproposed but none have been successfully operated, on account ofdlfflculties of cross modulation between the channels, distortion andthe like.

This system employs the wide band method of frequency modulationdescribed in my U. S. Patent #1,941,069 and preferably the method oftransmission described in my U. S. Patent #1,941,068. With the system ofmultiplexing ,of the present invention interference between the channelsis eliminated to an extent that multiplex transmission and reception ofbroadcast programs can now be successfully practiced. Simultaneoustransmission of two services such as the present sound program togetherwith a facsimile service or the transmission of the same sound programon separate channels for producing perspective broadcasting andreproduction are also practicable.

Referring now to the figures which form a part of this specificationFig. 1 represents the basic transmitting arrangement and Fig. 2 one formof the multiplexing system for use in conjunction with it. Fig. 3illustrates the general arrangement of the receiving system which isused when the system of Fig. 2 is employed. at the transmitter. Fig'. 4shows an improved form of multiplex system for use in conjunction withthe transmitter of Fig. 1 instead of the arrangement of Fig. 2. Fig. 5illustrates the receiving arrangements for use when the system of Fig. 4is used at the transmitter. Fig. 6 shows a characteristic curve of someof the operating features of the system which will be referred tohereinafter at length.

Referring now to Fig. 1, I represents a constant frequency oscillator,and 2 an amplifier of the output of this oscillator with a resistance 3in its plate circuit which is small in comparison with the impedance ofthe tube. 4 and 5 are likewise amplifiers of the current produced by themaster oscillator I. 42 is a transformer for differentially modulatingthe screen grid voltages of the amplifiers l and 5 by the signalingcurrent which is applied to the' primary of the transformer thru theamplifying system 3l-4l. 43 and 44 are by-pass condensers for shuntingbe hereinafter explainedthe two halves of the secondary of thetransformer 42, with respect to the high frequency currents of themaster oscillator frequency. 6 and 1 are inductances whose impedance forthe frequency of the master oscillator is small compared to theimpedance of the tubes 4 and 5. 8 and 9 are condensers whose reactancesfor the master oscillator frequency are equal to the reactances of theinductances 6 and 1. III is a small inductance whose natural frequencyis high compared to the frequency of the master oscillator. II is acondenser for balancing out the reactance of the coil I0. I2 is anadjustable resistance and I3 an amplifier connected across thisresistance. The plate circuit of this amplifier is connected to theresistance 3 in the output circuit of the amplifier 2. I4 is anamplifier for the combined outputs of thetubes i3, and I 5, I 6 and I1represent means for correcting the asymmetry of the transformer 3, 1,ill with respect to the 'two side bands as describedin my U. S. patentapplication Serial No. 40,542, filed September 14, 1935, Patent No.2,063,074, issued December 8, 1936. I8 and I9 are amplifiers of theoutput of this system and a frequency doubler. 2| is a filter for thepurpose of eliminating the fundamental frequency applied to the doubleras described in my U. S.

2 and patent application Serial No. 40,543, filed September 14, 1935, 22is an amplifier of the doubled frequency, 23a second doubler and 24 asecond filter for the purpose described above.

25 represents an amplifier, 26 a frequency doubler which may consist ofa series of doublers or tripplers, 21 the power amplifier and 28, 29 theradiating system. 30 represents the input to the modulating system ofthe transmitter, 3| an amplifier forthe modulating current, 33, 34

-a balanced push pull amplifier and 35 an output transformer feeding thecorrection system 36,

31. 38 is an amplifier feeding a second balanced push pull amplifier 40,ll thru the transformer 39. The output of the tubes 40, 4| is suppliedto the modulator tubes 4 and 5 thru the transformer 42. The generalarrangement so far is substantial- 1y along the lines laid down in my U.S. Patent #1,941,068 and the improvements described in my U. S.applications for patents Serial Nos.

40,542 and 40,543. Certain changes in design, however, in this part ofthe system which are essential to successful multiplex operation willReferring now to e'""ifiultiplex channel. ar-

rangementsshm g. 2, represents the input of channel #1.

is an amplifier for the currents of this channel and 41 a low passfilter to confine the frequencies to be transmitted over this channel toits designed width. 40 is a mix-.

Referring now to channel #2, 49 represents the input thereto and 50 anamplifier for the currents thereof. 52 represents an oscillator,usuallyof a superaudible frequency, but at least of a frequency which isthe sum of the highest frequency to be transmitted on channel #1 and thehighest modulating frequency to be applied to channel #2. Thru thecoupling transformer 53, this frequency is applied to the input of apush pull amplifier 54, 55 the output of which is cumulatively fed intothe band pass filter 58. The output of the amplifier 50 is supplied thruthe transformer 5| to modulate the plate voltage of the amplifier 54, 55thereby producing amplitude modulations upon the current of thefrequency of the oscillator 52. The band pass filter in this arrangementis made wide enough to pass both side bands. The modulated current whichpasses thru the filter 58 is supplied to the mixer tube 59 and combinedthereby with the currents of the output of the mixer tube 48 of channel#1.

The aforegiven description with respect to channel #2 applies exactlywith respect to channel #3 with the exception that the frequency of theoscillator 64 of channel #3 is chosen to be at least the sum of theoscillator frequency of channel #2 plus the sum of the modulatingfrequencies of channels #2 and #3. The input to the channel is suppliedat El and the output, thru the ,mixer tube II is combined with theoutputs of the other two channels for application to the main modulatingsystem of the frequency modulated transmitter at 30.

It is, of course; obvious that more channels operating on the sameprinciple may be added.

Referring now to Fig. 3, 18-, I9 represents the receiving antenna, anamplifier for the received current, 8I--86 the rectifiers, oscillatorsand amplifiers of a two intermediate frequency superheterodyne. 81 is acurrent limiter, 88 a filter for removing the harmonics produced by thelimiter. 89 is an amplifier for the second-intermediate frequency. 90,92, 94 and SI, 93, 95 are selective networks of the type de scribed inmy U. S. Patent #1,941,069 for converting the variations in frequencyinto amplitude variations. Connected in parallel with the reactancecombinations 92, 94 and 93, 95 are two high resistances 9B, 91 for thepurpose of straightening the characteristics of the combinations toinsure linear conversion of the frequency variations into amplitudechanges.

98, 99 are amplifiers whose input circuits are connected across thereactance combinations 92, 94 and 93,95 and whose output circuits areaperiodically coupled to the two diode rectifiers I04 and I05. In serieswith the output of the two amplifiers 9B, 99 and the two rectifiers I04,I05 are two high resistances I00 and IM for the purpose or securingstraight line rectification.

.shift within the working range.

Two blocking condensers I02 and I03 isolate the rectifiers from thedirect current plate supply of the rectifiers. I05 and I0! are chokecoils sufficiently large to block the intermediate frequency applied tothe rectifiers but not large enough to interfere with the fiow of thesuperaudible modulating currents. v I08 and I09 are a pair of equaltransformers which are substantially fiat over the range of thefrequencies occupied by the three channels combined at point 30 inFig. 1. The primaries of these transformers are shunted by resistancesH0 and III respectively. The secondaries of I08 and I09 are cumulativelyconnected for frequency modulation and connected to the grids of threeamplifier tubes, H2, H6 and I21. The output of H2 passes thru a low passfilter H3 designed to pass the frequencies of the first channel, thru anamplifier H4 and thence to the output circuitof channel #1. The outputof the amplifier H6 passes to a band pass filter I I! designed to passthe frequencies utilized in channel #2, and thence to an amplifier H8.The output of this amplifier is supplied to a rectifier I 2| thru theresistance I 20 and capacity H9. The output transformer I24 is connectedto the rectifier thru a choke I22 with a parallel resistance I23. Anamplifier I25 for the rectified current supplies the output of channel#2.

The description as given above with respect to channel #2 appliesequally well with respect to channel #3 except, of course, thatthe bandpass filter I28 is designed to handle the frequency employed at thetransmitter in signaling over channel #3.

Having now described the transmitting and receiving apparatus of Figs. 1and 2 their manner of operation will now be explained.

Suppose that the transmitter is designed to give a total swing offrequency of 150,000 cycles of the radiated wave which may be some ultrahigh frequency value. The design is carried out along the same lines asthose laid down in my U. S. Patents #1,941,068 and #1,941,069 and myapplications for U. S. patents Serial Nos. 40,542 and 40,543. There are,however, certain modifications. One of them is that, as the frequenciesof modulation, are much higher than merely the audible range, it isimportant that the width of the transformers and filters in thefrequency modulation system be expanded to take into-account thesehigher frequencies without producing a limitation of amplitude or phaseA second modification lies in the use of balanced push pull amplifierstages in all parts of the correction system when high levels exist. Thereason for this is as followswhe re high levels exist, as for example at33, 34 just preceding the correction net work 35, 31, a form of crossmodulation between channels takes place. the amplifying tubes acting asrectifiers of the superaudible frequency currents. As a result of thisthere is produced directly in the low frequency channel a current whichcorresponds in frequency to the modulations impressed upon thesuperaudible frequency channels and this current therefore modulates themain channel directly along With the proper modulation for that channel.This effect occurs even when the amplifying tubes in the correctionsystem are operated at relatively low levels. It is eliminated by thebalanced push pull amplifier because the-currents resulting from therectification of the superaudible currents have such phase relation Thisis caused by v the modulating current set at such value by adjustment ofthe amplification of tube 46 that the frequency swing of the radiatedwave is 50,000 cycles per second. This transmitted wave is of the sametype as that described in my U. S. Patent #1,941,069.

Referring now to the operation of channel #2 assume for the moment thatthe modulation is cut off from channel #1. By adjustment of the inputlevel to the amplifier 50 the modula tion applied to the plates of theamplifiers 54, of the superaudible frequency is raised to a pointsufficient to give "substantially complete modulation of thatfrequency.By adjustment of the amplification of tube 59 the voltage applied to theinput of the transmitter modulation systemis raised to a pointsufficient to give 25,000 cycles complete swing of the modulated wavewhen no modulation is applied to thesecond channel. Under theseconditions there occurs in the radiated wave a swing of 25,000 cycles atthe rate of the frequency of the oscillator 52 of the second'channel.The extent of this swing of course depends on the amplitude of thevoltage output of the tubes 54 and 55. Hence when the plates of thesetubes are modulated, for full modulation the output voltage variesbetween double the unmodulated voltage and zero. Hence the swing of thetransmitted wave varies between.50,000 cycles and zero, the swing stillcontinuing at the rate of the frequency of the oscillator 52 but varyingin extent at a rate corresponding to the frequency of the modulationap-' plied to the input of channel #2 at 49.

An identical action occurs with respect to channel #3, when it isoperated alone. By adjustment of the input level at El the modulation ofthe amplifier 66, 61 is adjusted to be substantially complete. Similarlyby adjustment of the amplification of 'II the swing of the transmitted.wave is adjusted to be 25,000 cycles when no modulation is applied tothe input of channel #3 and to vary between the limits of 50,000 cyclesand zero when modulation is applied. The swing is, of course, at therate of' the oscillator 64 of the third channel.

Referring now to the receiver the operation is as follows. The number Ior main channel functions in the same manner described in my U.

S. Patent #1,941,069. The received signal is amplified by thesuperheterodyne receiver 'I886,

supplied to the current limiter 01, passed thru the filter 88, and thenamplified and supplied to the selective system 00-91 which convertsthefrequency variations of the signal into amplitude rent of channel. #1 isof an audible frequency. The number of variations of the frequency oc--curring in the transmitted wave in response to the modulations on thischannel corresponds to the frequency of the modulating current and the.

extent of the deviations in frequency is proportional to the amplitudeof the modulating current. Hence the variations in amplitude created bythe action of the selective system -91 are of an audible frequency whichcorrespond to the frequency of the modulating current of channel #1.After rectification by the rectifiers I04, I05 they may be supplied tothe speaker in the ordinary way.

Referring now to channel #2 the operation is somewhat more involved.From the antenna to the input of the selective system 909I the receiverperforms the same function as in the case of the main channel. However,because of the double modulation involved in the wave transmitted onchannel #2 the action of the selective system is to convert thedeviations of frequency in the received wave into superaudible'variations of amplitude in which the amplitude of these variations arenot constant but vary in frequency and amplitude in accordance with themodulating current of channel,#2. When the above current is rectified bythe rectifiers I04 and I05 there appears in the output circuit a currentcorresponding in frequency to the oscillator frequency of channel #2which varies in amplitude in accordance with the modulations impressedtaneously at the levels which have been considered here. Since theaction of the transmitter and receiver both is linear in every respectthe currents co-exist in all parts of the system without crossmodulation and the swings of frequency of the modulated wave are acombination of the individual swings of the various channels, the totalswing of 150,000 cycles occurring when all three channels aresimultaneously frilly modu- -upon the input to that channel. This bandof frequencies is then passed thru the filter III,

lated and the phases of the modulations such as to be additive. As apractical matter this so rarely occurs that it is possible to run eachof the channels at a higher level than its one third value with aproportionate improvement in the noise level. The exact settings arereadily determined experimentally. It islikewise true that where varioustypes of transmissions are employed the permissible noise levels will bedifferent so that a greater percentage .of the total swing may beassigned to 'the channel most subject to disturbance with acorresponding lesser percentage assigned to those services leastaffected by extraneous disturbances.

It is, of course obvious that While channel #1 has been referred to asan audible frequency channel and the other two as superaudible frequencychannels that the choice of the different auxiliary frequencyoscillators depends on the Referring now to Fig. 4 there is shown asecond method of multiplexing which, while more complicated than thesystem of Figs. 1 and 2 has sub- 5 I range of frequencies which it isdesired to trans-- stantial advantages over that system. In this systemthe modulations of the signals to be transmitted are impressed on thesuperaudible frequency channels not as modulations of the amplitude ofthe currents of these channels but as variations in the frequency. Ineffect it represents a frequency modulation within a frequencymodulation and the method has great ad- -vantages in freedom fromcertain forms of cross modulation between the channels.

Referring now to Fig. 4, there is shown a multiplex system of twochannels whose combined output it is intended to supply to the input3110f the transmitter of Fig. 1. Channel #1 is supplied at I40 thru anamplifier MI, and a low pass filter I42 to confine the frequenciesadmitted to this channel to its designed range and a mixing tube. Thischannel is the same as channel #1 of the system already described.

Channel #2 in the present system differs from channel #2 of the firstsystem in that the frequency of this channel is modulated instead of itsamplitude. On account of .the fact that the frequency of the channel isnecessarily low and that a frequency swing representing a considerablepercentage of unmodulated value of frequency is required an expedient isresorted to in order to obtain an undistorted wave. I44 to I63illustrates the master oscillator and frequency modulating arrangementsof a transmitter which are identical with the arrangement of the maintransmitter illustrated in Fig. I by I- I8. I64 represents a frequencydoubler, I65a filter, I66 a second doubler, I61 a second filter and I68a frequency multiplier of as many stages as necessary to secure thedesired frequency change.

The action of all this part of the system is iden-v tical with thecorresponding part of the main transmitter. The output of the frequencymultiplier is supplied to a rectifier I69, where it is heterodyned down,by means of the oscillator I10 to the frequency of channel #2. HI is aband pass filter for eliminating disturbing currents outside the band,I12 an amplifier and I13 a mixing tube.

The modulation of the second channel is applied at I14 to the amplifierI15 which feeds the correction system I11, I18 thru the transformer I16.The voltage developed across the condenser I18 is then amplified by thetubes I19, I and applied to the screen grids of the modulating tubes I46and I41.

I The operation of the transmitting system is as follows. The main ornumber one channel functions in the same way as the main channel of thearrangement of Figs. 1 and 2. The maximum swing for this channel,assuming as in the preceding case a maximum swing of the transmitter of150,000 cycles, should be set at 75,000 cycles.

The operation of channel #2 is as follows. The oscillating andmodulating circuits I44I63 together with the correction system and itsamplifiers I15-I8I produces a high frequency current whose phase isshifted-by an amount directly proportional to the amplitude of themodulating current and inversely proportional to its frequency in themanner already described. By means of the frequency multiplying systemI64-I68 this frequency is multiplied up to some high value in order toproduce the required degree of modulation. This frequency may be of theorder of 10,000,000 cycles or more. Assuming a value of 10,000,000 theoscillator I10 may be set to a frequency of 10,025,000 cycles. By meansof the'rectifier I69 the output of the frequency multiplier system isheterodyned down to 25,000 cycles. Any variation in frequency in the10,000,- 000 cycle current is therefore reproduced numerically in the25,000 cycle current. Since a 10,000 cycle deviation in the 10,000,000cycle current is readily obtainable it follows that the 25,000 cyclecurrent may be swung from 15,000 cycles to 35,000 cycles. The output ofthe rectifier is passed thru a band pass filter to exclude extraneousfrequencies and supplied thru the mixer tube I13 to the input 30 of themodulation system of the transmitter. By adjustment of the level of thiscurrent the modulation of the transmitter is adjusted to be 75,000cycles total swing. The transmitted wave, therefore, varies in frequencycontinuously over a range of 75,000 cycles at a rate which is 25,000times per second when the second channel is unmodulated and which variesbetween the limits of 15,000 times and 35,000 times-per second when thesecond channel is fully modulated. The number of times the rate ofmodulation of the transmitter varies between 15,000 times and 35,000times per second correspond-s to the frequency of the modulation appliedto the input of channel #2. It should be noted that the extent of theswing of the transmitted wave does not vary during the modulation of thesecond channel but is constant at 75,000 cycles per sec ond. Since theswing remains constant and only the frequency at which it isaccomplished, is changed, the tendency to produce cross modulationbetween channels is greatly reduced. In practice this advantage is fullyrealized.

Referring now to Fig. 5 there is illustrated the general arrangement ofthe receiving system for this method of multiplexing. In this figureI90, I9I. represents the receiving antenna, I92 an amplifier for thereceived current, I93-I98, the rectifiers, oscillators and amplifiers ofa two intermediate frequency superheterodyne. I99 is a current limiter,200 a filter for removing the harmonies of the limiter and 20I anamplifier for the second intermediate frequency. 202, 204, 206 and 203,205 and 201 are selective networks of the type described in the patentheretofore referred to for converting the variations of frequency intovariations of amplitude. Connected in parallel with the reactancecombinations 204, 20,6 and 205, 201 are two resistances 208, 209 for thepurpose of straightening out the reactance characteristics to insurelinear conversion of the frequency variations into amplitude changes.2I0 and 2 are amplifiers whose input circuits are connected across thetwo reactance paths and whose outputs supply the two linear rectifiers'2I6, 2I1. 222 and 223 represent two balanced transformers whosesecondaries are connected to act cumulatively for frequency variations.224 is an amplifier for supplying the frequencies of channel #1 to a lowpass filter 225 which eliminates undesired frequencies from the output221 of channel one. 228 is an amplifier for the current-s of the secondchannel, 229 a filter, 230 an amplifier, 23I a current limiter, 232 afilter for removing harmonics produced in the limiter and 233 anamplifier for the limited current. The remainder of the system from 234to 255 represents the same kind of selective networkfor convertingfrequency changes into amplitude changes and the same kind of rectifyingarrangement as is shown by 202223 with'the exception of course that itis designed to handle a current of variable frequency of a much lowerorder. The output of the two transformers 254 and 255 are similarlyconnected cumulatively for frequency variations and connected thru a lowpass filter- 256 to the output 251 of the second channel.

The operation of the receiving system is as follows ghannel #1 operatesin the normal manner and" the selective system 202- 209 converts thefrequency variations into amplitude variations, which afterrectification to produce the original modulating current of channel #1are separated out by the filter 225 and pass to the output 221 of thatchannel.

The second modulation of frequency of the received wave is likewiseconverted into amplitude variations by the selective systems 202-209 andresults in an amplitude variation of constant amplitude of the voltageapplied to the inputs of the amplifiers 2) and 2, the frequency of whichvaries between 15,000 and 35,000 cycles per second. When rectified bythe rectifiers M6 and 2H there results a frequency modulated current ofconstant amplitude varying in frequency from 15,000,

255 is then passed thru the low pass filter 256 to the output of thechannel ,251.

The same advantages which were referred to in connection with thetransmitter regarding the v freedom from cross modulation betweenchannels are present in the receiver for the same reasons, that is, theuniform amplitude of the superimposed channel.

The explanation of the fact that the system will operate moreefiectively with a given amount 'of power than if the power were dividedamong receiver increases, the signal levelremaining unchanged. When theincoming radio frequency voltage of the signal is above a certain ratiowith respect to the noise level, the relation between the signal and thenoise levels in the output of the receiver is a strictly quantitativeone-that is, the noise level in the output of the receiver isinverselyproportional to the strength of the signaling current in the antenna.When, however, the noise and signal voltages in the antenna approachequality, any further slight r decrease in the strength of the receivedsignal results, in a great increase in the strength of the disturbancesin the output of the receiver. This is clearly shown in the curve. Forexample. if a .single simplex channel is producing a voltage in that ofeither one alone will be received. This is illustrated by 0N with acorresponding noise level of OP. The extent of the improvement and thereason for it appears at once from the curve. The advantage obviouslybecomes the greater the more simplex transmitters that are combined intoa single multiplex system.

While in this specification there has been shown two types of modulationonly applied to the auxiliary channels, it is, of course clear thatother types, such as phase, or single side band, or any other form maybe used where desired without departing from the spirit of theinvention.

I claim:

1. A frequency-modulated multiplex signaling system comprising aplurality of channels, each channel consisting of a separate band offrequencies, said system comprising means for causing the said channelsto frequency-modulate the transmitted wave, and means for causing thefrequency deviation of the transmitted wave produced-by each channel tobe substantially greater than the frequency range of audibility, so asto reduce cross-modulation between the channels.

2. A receiver for a multiplex frequency modulated signaling system asspecified in claim 1, said receiver comprising means for amplifying thereceived current, means for removing from said current the efiects ofamplitude variations,

means for translating the frequency variations of the received currentinto changes in amplitude,

said means comprising a selective system adjusted to produce fullamplitude changes in the amplified current only in response to the widefrequency swing of the transmitted wave, a detecting device for saidamplitude modulated currents and means for separating out thefrequencies of the respective channels from the output of the saiddetecting device.

3. A transmitter for a multiplex signaling system comprisinga pluralityof sources of signaling current, means for generating a wave of thefrequency to be transmitted, a source of superaudible frequency, meansfor modulating the super-audible frequency by one of saidsignalingcurrents, means for causing another of said signaling currents and thesaid modulated superaudible frequency current to modulate the frequencyof the radiated wave, and means for causing the frequency deviations ofthe transmitted wave produced. by each of said. currents to besubstantially greater than the frequency range of audibility so as toreduce crossmodulation between said currents.

4. A transmitter for a multiplex signaling system comprising ,aplurality of sources of signaling current, means for generating a waveof the frequency to be transmitted, a source of superaudible frequency,means for modulating theampiitude of the super-audible frequency by oneof said signaling currents. means for causing another of said signalingcurrents and the said modulated super-audible frequency currentv tomodulate the frequency of the radiatedfwave, and means'for causing thefrequency deviations of the transmitted wave produced by each of saidcurrents to be substantially greater than the frequency range ofaudibility, so as to reduce cross modulation between said currents,

5. A transmitter for a multiplex signaling system comprising atpluralityof sources of signalingcurrent, means for generating a wave of thefrequency to be transmitted,- a source of superaudible frequency, meansfor modulating the'frequency of this super-audible.frequency by one of 7said signaling currents means for causing another of said signalingcurrents and the said modulated super-audible frequency current tomodulate the frequency of the radiated wave, and means for causing thefrequency deviations of the transmitted wave produced by each of saidcurrents to be substantially greater than the frequency range ofaudibility, so as to reduce cross modulation between said currents.

6. A receiver fora multiplex signaling system the received wave of whichis generated by the I transmitter of the character described in claim 4,said receiver comprising means for eliminating amplitude variations fromthe received wave,

means for translating the deviations in frequencyv the received wave ofwhich is generated by the transmitter of the character described inclaim 5, said receiver comprising means for eliminating amplitudevariations from the received wave, means for translating the deviationsin frequency corresponding to the modulations impressed upon thetransmitted wave by the signaling current of the first channel intocurrents corresponding in amplitude and frequency to the amplitude andfrequency of the first signaling current source, means for translatingthe deviations of frequency produced by the modulating current of thesecond channel into super-audible currents of varying frequency, andmeans for converting the variations in frequency into currentcorresponding in amplitude and frequency to the amplitude and frequencyof the second signaling source.-

8. A transmitter for a multiplex signaling system comprising a pluralityof sources of signaling current, means for generating a wave of thefrequency to be transmitted, a source of superaudible frequency, meansfor modulating the frequency of this super-audible frequency by one ofsaid signaling currents and means for causing another of said signalingcurrents and the said modulated super-audible frequency current tomodulate the frequency of the radiated wave.

EDWIN H. ARMSTRONG.

